(1) Field of the Invention
The present invention relates to liquid crystal display devices. This invention more specifically relates to a novel backlight, which may provide for highly bright and efficient liquid crystal display devices. Even more specifically it relates to displays that use the mechanism of light recycling to provide brighter appearance.
(2) Background Information
The demand for liquid crystal displays (LCDs) has increased substantially in recent years with the proliferation of computer technology and portable electronic applications. LCDs are further considered by many to be the most promising technology for meeting the demands of future flat panel display applications.
In general, as shown in FIG. 1, a conventional LCD includes the following three basic components: (i) a backlight 10 for producing a plane of relatively uniform intensity light; (ii) an electrically-addressable array 20 of spatial-intensity modulating elements for modulating the spatial intensity of the plane of light transmitted therethrough; and (iii) an array of absorptive color filtering elements 30 in registration with the array of spatial-intensity modulating elements, for spectrally filtering the intensity of modulated light transmitted therethrough to form a color image.
Backlight 10 generally includes one or more light sources 11 (e.g., thin fluorescent tubes), a light guide 12, at least one diffuser element 13 positioned optically downstream of light guide 12, and a highly reflective (e.g., white) film 14 positioned optically upstream of light guide 12, which serves to reflect light back into the display. Electrically addressable array 20 generally includes a linear polarizer 21, a layer that includes the addressing circuitry (e.g., thin-film transistors (TFTs), capacitors, and buslines) 22, a liquid crystal (LC) cell 23, which is often in a 90xc2x0 twisted nematic configuration, and one or more glass substrates 24. Filtering array 30 typically includes a pixilated absorptive color filter 31, another linear polarizer 32 and one or more glass substrates 33.
It is well known that one of the principle shortcomings of a conventional LCD is poor light transmission efficiency (i.e., a high percentage of the light generated by backlight 10 is absorbed by the various LCD components). The light efficiency of a typical, conventional LCD panel is generally about 5% to 10%. The light transmission efficiency of conventional LCD panels tends to be substantially degraded by the following factors: (i) absorption of light by two absorption-type polarizers, (ii) absorption of light by absorption-type spectral filters, (iii) absorption of light reflected off TFTs and/or wiring, (iv) absorption of light by the black-matrix used to spatially separate the subpixel filters, and (v) Fresnel losses owing to mismatching of refractive indices between layers. As a result, it tends to be difficult to produce highly bright images from a conventional LCD panel without using ultra-high intensity backlighting systems, which require substantial electrical power input and generate a significant amount of excess heat.
Recently, Faris, in U.S. Pat. No. 6,188,460 (hereinafter referred to as the Faris patent), disclosed a LCD employing a systematic light-recycling scheme in order to eliminate the light energy losses associated with conventional displays. The Faris patent is fully incorporated herein by reference. In the optical recycling scheme disclosed therein, polarized light is transmitted from the backlight to those components in the LCD where spatial intensity modulation and spectral filtering occurs. Light that is not transmitted to the display surface (i.e., to a viewer) tends to be reflected back (rather than being absorbed) along the projection axis into the backlight for recycling and retransmission through the backlight for reuse by both the same and neighboring subpixels. The end result of the light recycling process is a brighter display since it converts a higher portion of the light into a usable form than conventional systems.
Notwithstanding the substantial improvements disclosed in the Faris patent, there remains a need for an improved LCD that is capable of efficiently producing high brightness color images. More particularly there is a need for a backlight that provides for highly efficient light recycling.
In one aspect the present invention includes a backlight for a liquid crystal display employing light recycling. The backlight includes a light source, a light guide fabricated from a substantially non-absorptive material, the material being selected from the group comprising acrylic, polycarbonate, and poly (methyl-methacrylate), wherein the light guide absorbs less than 5% of reflected light energy incident thereon, and a reflective layer fabricated from a highly reflective material, the material being selected from the group comprising aluminum, silver, barium sulfate, magnesium oxide, and organic materials, wherein the reflective layer reflects at least 95% of the light energy incident thereon. In a further aspect, this invention includes a liquid crystal display including an electrically addressable array including a liquid crystal cell, a substantially non-absorptive filtering array, a broadband polarizer, and the backlight described hereinabove in this paragraph.
In another aspect this invention includes another embodiment of a backlight for a liquid crystal display employing light recycling. The backlight includes a light source, a bundle of optical fibers, the optical fibers including an optically upstream side and an optically downstream side, the optical fibers further including a cladding material, and a reflective layer fabricated from a highly reflective material, the material being selected from the group comprising aluminum, silver, barium sulfate, magnesium oxide, and organic materials, wherein the reflective layer reflects at least 95% of the light energy incident thereon. The optical fibers are further configured to receive light from the light source and distribute the light to the reflective layer. In a still a further aspect, this invention includes a liquid crystal display including an electrically addressable array including a liquid crystal cell, a substantially non-absorptive filtering array, a broadband polarizer, and the backlight described hereinabove in this paragraph.
In still another aspect, this invention includes a method for fabricating a backlight for a liquid crystal display. The method includes providing a light source, providing a reflective layer fabricated from a highly reflective material, the material being selected from the group comprising aluminum, silver, barium sulfate, magnesium oxide, and organic materials, wherein the reflective layer reflects at least 95% of the light energy incident thereon, and providing a bundle of optical fibers, the optical fibers including an optically upstream side and an optically downstream side, the optical fibers further including a cladding material. The method further includes positioning the optically upstream side of the optical fibers in operative engagement with the light source and positioning the optically downstream side of the optical fibers in operative engagement with the reflective layer. To fabricate a LCD including a fiber optic backlight, the method may further include superposing a substantially non-absorptive spectral filtering array with the reflective layer and superposing an electrically addressable array including a liquid crystal cell with the reflective layer.